The invention relates to melt processible fluoropolymer compositions, in particular compositions that comprise a melt processible fluoropolymer comprising repeating units derived from tetrafluoroethylene (TFE) and hexafluoropropylene (HFP). Such copolymers are called xe2x80x9cFEPxe2x80x9d.
FEP fluoropolymers have been known for a long time (U.S. Pat. No. 2,946,763), and are commercially available. FEP fluoropolymers are perfluorinated thermoplastic fluoropolymers that have excellent heat resistance and chemical resistance. FEP fluoropolymers also have a low dissipation factor (EP-A-423 995). Due to all of these properties FEP polymers are of interest for use as an insulating material for cable-wire insulation, in particular for what are known as plenum wire cables, used for example in LANs (local area networks). The processing speeds for producing insulating plenum cables are very high. FEP polymers that can be used in producing such plenum cables are therefore generally those which permit processing at high shear rates without loss of the necessary mechanical properties.
FEP polymers which have a broad molar mass distribution ensure relatively fast processing at relatively high shear rates (DE-A-26 13 795, DE-A-26 13 642, EP-A-88 414, EP-A-362 868). Modification with another comonomer (DE-A-27 10 501, EP-A-75 312), such as perfluoro vinyl ethers, yields retention of the necessary mechanical properties. To generate high extrusion speeds while retaining a smooth melt surface, nucleating agents are often added to the polymeric materials to suppress, and/or shift the occurrence of xe2x80x9cshark skinxe2x80x9d (melt surface instability, giving a rough surface) to higher shear rates(U.S. Pat. No. 5,688,457).
Besides the formation of xe2x80x9cshark skinxe2x80x9d at high shear rate, the tendency of perfluorinated thermoplastics to form die deposits has to be considered. These die deposits are processing condition dependent, and take effect in different ways. In fast extrusion procedures, such as cable-wire insulation, large accumulations of die deposits separate from the die and cause break-off of the melt cone and thus interruption of the production, and also interruption of the continuous cable. High processing temperatures promote die deposits, and at these temperatures the FEP products decompose more rapidly, as becomes apparent through discoloration and molecular degradation. This thermal instability is attributable to unstable end groups, HFP diads in the main polymer chain (EP-A-150 953) and metal contamination. The decomposition reaction of the thermally unstable end groups has been described in xe2x80x9cModern Fluoropolymersxe2x80x9d, Ed. John Scheirs, Wiley and Sons 1997, page 228. For this reason thermally unstable end groups, including COOH, CONH2 and COF groups, are preferably converted into thermally stable end groups by fluorination (GB-A-1 210 794, EP-A-150 953, EP-A-222 945) or by a stabilization process in the presence of water vapor (DE-A-26 13 795, DE-A-26 13 642). The amounts of die deposits can be minimized by preparing FEP materials with stable end groups, combined with high purity with respect to metal ions and narrow molecular weight distribution (German Patent Application 199 03 657.8 of Jan. 29, 1999, corresponding to PCT/EP00/00528 of Jan. 24, 2000). However, this high purity is accompanied with increased purification costs, and a narrow molecular weight distribution, which further promotes the onset of xe2x80x9cshark skinxe2x80x9d.
When FEP polymers are melt extruded, high extrusion speeds increase deposits on the die. These deposits, known as die deposits, accumulate as time passes and break away from the die when they reach a particular size. This results in damage to the final product or, in the case of break-off of the melt, to interruption of production with other serious consequences. After a break-off production has to be interrupted until a new cable has been threaded into the die. The break-off also limits the length of the cable, thus producing unnecessary waste material in the twisting of a number of cables of different length. Die deposits of this type are therefore regularly removed from the die during processing, but this removal is almost impossible during high-speed processing, such as cable-wire insulation, and particularly in this application it had to be accepted that frequent break-offs of the melt cone would occur.
The present inventors have found that it would thus be desirable to reduce the number of times a melt cone breaks off in the extrusion of FEP polymers, in particular at high speed. Preferably, this problem is solved without sacrificing the resulting mechanical properties. Desirably, the mechanical properties of the FEP polymers are further improved.
In accordance with the present invention there is provided a melt-processible perfluorinated polymer composition comprising
a) a melt-processible perfluoropolymer comprising
(i) from 80 to 98% by weight of repeating units derived from tetrafluoroethylene,
(ii) from 2 to 20% by weight of repeating units derived from hexafluoropropylene, and
(iii) from 0 to 5% by weight of repeating units derived from further comonomers other than tetrafluoroethylene and hexafluoropropylene, and wherein the proportion by weight of the repeating units derived from hexafluoropropylene units is greater than that of the repeating units of said further comonomers, and
b) from 0.01 to 5% by weight, based on perfluoropolymer a), of a high-molecular-weight perfluorinated polymer with a melting point at least 20xc2x0 C. above that of the fluoropolymer a).
The invention further provides a method of producing the above melt-processible composition, the use thereof in melt-extrusion, in particular to extrude insulation around a wire to produce an electrical cable. The invention also relates to an electrical cable having the melt-processible composition as an insulation.
The inventors have recognized that the actual problem is not the die deposit itself but excessive accumulation of the same and the release of relatively large accumulations, which finally leads to break-off of the melt cone during wire coating.
A mixture of the aforementioned melt-processible perfluoropolymer with a small proportion of a high-melting, i.e. having a higher melting point than the melt-processible perfluoropolymer, and high-molecular-weight fluoropolymer, i.e. having a higher molecular weight than the melt-processible perfluoropolymer, performs quite differently than known FEP melt-processible perfluoropolymers. Under the same conditions, there is very little accumulation of the die deposits which form, since they regularly break away from the die at very short intervals. Without intending to be bound by any theory, it is believed that a possible reason for this is the presence of minor non-uniformities in the melt, which may be brought about by the high-molecular-weight, higher-melting fluoropolymer which entrains the die deposits. The use of the mixture of the melt-processible perfluoropolymer with the higher-melting, higher molecular weight perfluoropolymer for high-speed cable-wire sheathing, for example plenum wire production, may reduce by a factor of 5 the number of break-offs observed of the melt cone. This ensures continuous production with fewer interruptions to production and longer cables.
The melt-processible composition of the invention generally also permits an improvement in mechanical properties in comparison with prior art FEP polymers.
For example the flexural fatigue strength (xe2x80x9cflex lifexe2x80x9d) of the melt-processible composition of this invention is often many times greater than that of known FEP copolymer products with the same melt flow index (MFI). In the art, high flexural fatigue strengths of FEP products have hitherto been achieved by modification with perfluoro alkyl vinyl ethers (PAVEs). However, the dipole moment of PAVEs and the high dissipation factor associated with this makes these materials disadvantageous, particularly for high-frequency cable applications.
The melt-processible perfluorinated polymer composition according to the invention, may achieve comparable flexural fatigue properties without the need for any modification with PAVEs. Of course, if desired modification with PAVEs may be utilized to further improve flexural fatigue strength and elongation at break at high temperatures.
In accordance with the present invention, the melt-processible perfluorinated polymer composition comprises a melt-processible perfluoropolymer comprising from 80 to 98% by weight of repeating units derived from TFE, between 2 and 20% by weight, preferably between 7 and 16% by weight of repeating units derived from HFP and between 0 and 5% by weight of further comonomers other than TFE and HFP and wherein the proportion by weight of the HFP derived repeating units is larger than that of the repeating units derived from the further comonomers. Suitable further comonomers include PAVEs. Examples of suitable PAVE monomers include those corresponding to the formula:
CF2xe2x95x90CFxe2x80x94Oxe2x80x94Rfxe2x80x83xe2x80x83(I)
wherein Rf represents a perfluorinated aliphatic group that may contain one or more oxygen atoms. Preferably, the perfluorovinyl ethers correspond to the general formula:
CF2xe2x95x90CFO(RfO)n(Rxe2x80x2fO)mRxe2x80x3fxe2x80x83xe2x80x83(II)
wherein Rf and Rxe2x80x2f are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10, and Rxe2x80x3f is a perfluoroalkyl group of 1-6 carbon atoms.
Examples of perfluorovinyl ethers according to the above formulas include perfluoro-2-propoxypropylvinyl ether, perfluoro-3-methoxy-n-propylvinyl ether, perfluoro-2-methoxy-ethylvinyl ether, perfluoromethylvinyl ether (PMVE), perfluoro-n-propylvinyl ether and CF3xe2x80x94(CF2)2xe2x80x94Oxe2x80x94CF(CF3)xe2x80x94CF2xe2x80x94Oxe2x80x94CF(CF3)xe2x80x94CF2xe2x80x94Oxe2x80x94CFxe2x95x90CF2.
Preferably, the melt-processible perfluoropolymer of the melt-processible perfluorinated polymer composition has a melt flow index (MFI) in grams per 10 minutes (g/ 10 min) of more than 0.5, preferably at least 2, more preferably at least 5 when measured at 372xc2x0 C. at a load of 5 kg. The melting point of the melt-processible perfluoropolymer is generally 230 to 280xc2x0 C., preferably 240 to 270xc2x0 C.
The melt-processible perfluorinated polymer composition further contains from 0.01% by weight to 5% by weight, preferably 0.05 to 0.5% based on the weight of the melt-processible perfluoropolymer of a high-molecular weight, higher melting perfluoropolymer. The higher melting perfluoropolymer typically has a melting point of at least 20xc2x0 C. above, preferably at least 30xc2x0 C., and more preferably at least 40xc2x0 C. above the melting point of the melt-processible perfluoropolymer. The higher melting perfluoropolymer that is contained in the composition as a minor component, typically will have a MFI measured at 372xc2x0 C. and with a load of 5 kg of not more than 0.5. Preferably, the higher melting perfluoropolymer will have a melting point of at least 270xc2x0 C., preferably at least 290xc2x0 C. Examples of suitable higher melting perfluoropolymers include copolymers of TFE and PAVEs, e.g. as mentioned above.
In a preferred embodiment, the perfluoropolymers of the melt-processible composition of the invention will have fewer than 70, in particular fewer than 5, thermally unstable end groups per 106 carbon atoms. Thermally unstable end groups include COOH, CONH2 and COF groups. These can be readily converted into more stable end groups through fluorination as disclosed in GB-A-1 210 794, EP-A-150 953 or EP-A-222 945 or by a stabilization process in the presence of water vapor as disclosed in DE-A-26 13 795 or DE-A-26 13 642.
The fluoropolymers constituting the melt-processible composition of the invention may be prepared by any of the known polymerization methods including aqueous or nonaqueous polymerization.
The melt processible compositions may be prepared by mixing the melt processible perfluoropolymer and the high molecular weight, higher melting perfluoropolymer. In particular, the composition can be prepared by mixing the dispersions of the respective perfluoropolymers or alternatively, the composition can be prepared through seed polymerization or core-shell polymerization. For example, in a seed polymerization, the higher melting, high molecular weight perfluoropolymer may be used as a seed in an aqueous emulsion polymerization for making the melt processible perfluoropolymer. As a result of such a seed polymerization, a melt-processible composition according to the invention can be directly obtained. Similarly, in a core shell polymerization, the high molecular weight perfluoropolymer may be polymerized in the first stage of the polymerization and in a subsequent stage of the polymerization, the composition of the polymerization system may be changed to produce the melt-processible perfluoropolymer. Again, a melt-processible composition according to the invention may thus be directly obtained.
The melt-processible composition is particularly suitable for producing electrical cables wherein the composition of the invention serves as an insulator. Cables with a low dissipation factor may be produced, and such cables are thus particularly suitable for high-frequency applications (e.g. 100 MHz to 10 GHz) as for example with plenum wire cables, coaxial cables for transmitting for example a television signal and xe2x80x9ctwisted pairxe2x80x9d cables. To produce an electrical cable, the melt-processible composition of the invention is typically extruded around a central conductor. To produce a coaxial cable, an outer conductive element, for example, a metallic foil, a woven or braided composite wire or a drawn aluminum, copper or other metallic tube may be provided around the insulated cable. Typically, this outer conductive element will be encased in further protective insulation. Twisted pair cables are similar to coaxial cables in that a central conductor is surrounded by a low-loss insulation, except that a plurality, typically two, of such conductors are twisted together.
Analytical Methods:
The content of perfluorinated comonomers (U.S. Pat. No. 4,029,868, 4,552,925) and the number of end groups (EP-A-226 668, U.S. Pat. No. 3,085,083) are determined by IR spectroscopy. For this, a Nicolet Magna 560 FTIR is utilized. The total number of unstable end groups is calculated from the number of isolated and bonded COOH groups, CONH2 groups and COF groups. The total number of these end groups is in all cases given below.
The MFI gives the amount of a melt in grams per 10 min which is extruded from a holding cylinder through a die by the action of a piston loaded with weights. The dimensions of die, piston, holding cylinder and weights are standardized (DIN 53735, ASTM D-1238). All of the MFIs mentioned here have been measured with a die measuring 2.1 mm in diameter and 8 mm in length, using a superimposed weight of 5 kg and a temperature of 372xc2x0 C.
The flexural fatigue strength (xe2x80x9cflex lifexe2x80x9d) tests were carried out using a model 956, no. 102 device from Frank, built in 1967. Strips of film having a width 15 mm, a thickness of 0.3 mm, and a length of at least 100 mm were tested. Adhesive strips were used to hold a film sample of about DIN A5 size to the drum of a film cutter, a draw-knife system was put in place, and the cutting drum was rotated to produce strips at the preset knife separation. The strips of film were clamped into the screw clamps of the flexural fatigue (Frank) device and loaded with a suspended weight of 1529.6 g. The strips of film were flexed in the apparatus through an angle of 90xc2x0 in both directions at a folding frequency of 250 double flexures per minute until fracture occurred. A counter on the device recorded the number of double flexures until fracture. The flexural fatigue strength, or flex life, of a material was the average number of double flexures until failure for three samples.
The examples below describe the invention in more detail. Percentages and ratios are based on weight unless otherwise stated.